This invention relates to a printing apparatus and to a method of controlling the power supply thereof. More particularly, the invention relates to a printing apparatus for performing printing on a printing medium by a printhead having a plurality of printing elements, and to a method of controlling the power supply of this recording apparatus.
Printers for printing desired information such as characters and images on a sheet-like print medium such as paper or film are available as the information output devices of word processors, personal computers and facsimile machines, by way of example.
Various techniques are known for application to printing methods employed by printers. Ink-jet technology has become the focus of attention in recent years because of its ability to print on a print medium such as paper without contacting the medium, the facility with which it lends itself to color printing and the quiet with which a serial printing method is employed most widely as the ink-jet printing method because of the advantages of lower cost and smaller size. The serial printing method employs a mounted printhead for discharging ink in accordance with desired print information. Printing is carried out while the printhead is scanned back and forth at right angles to the direction in which the print medium such as paper is fed.
The widespread use of personal computers and digital cameras has become pronounced in recent years. In addition, in response to user demand, applications that make it possible to print photographs also have come into greater use with the proliferation of digital cameras and the like.
Improvements in the processing capability and processing capacity of image input devices such as digital cameras have been accompanied by the desire for better image quality and higher definition also in printers used as image output devices. In response to such need, many high-quality printers capable of producing a high-definition output equivalent to that obtained with photographic paper have been proposed.
Though ink-jet printers having the features mentioned above are being used widely as printers that provide an output of high image quality at low running cost. More recently, there has been a greater tendency toward raising the density and number of nozzles that serve as the printing elements in order to obtain a print output of higher definition while printing speed is maintained.
An ink-jet printer is usually provided with discharge-pressure generating sources such as heaters or piezoelectric elements in one-to-one correspondence with the discharge nozzles. If nozzle density is raised and the number of nozzles increased. Therefore, there is an increase in the load on the driving power supply that supplies power to the discharge-pressure generating sources such as heaters or piezoelectric elements.
If the load on the driving power supply increases, a problem which arises is a fluctuation in ink discharge performance caused by a drop in driving voltage. In particular, if a difference develops in ink discharge quantity and ink impact position precision in a case where there is a change in the number of nozzles driven simultaneously by print data, as when only one dot is printed and when a plurality of dots are printed simultaneously, this is reflected directly in the printed result as disturbance of the output image. This problem is not limited to ink-jet printers and arises also in other types of printers that have large numbers of printing elements.
A variation in driving voltage due to the number of nozzles driven simultaneously is influenced not only by the capacity of the power supply but also by the resistance of the wiring from the power supply to the discharge-pressure generating sources and by the common impedance. Accordingly, there has been proposed a printing apparatus in which the printhead and power-supply unit are provided on a carriage, which is scanned back and forth in the main-scan direction, for the purpose of suppressing a change in driving voltage by placing the power supply in close proximity to the discharge-pressure generating sources to shorten the wiring. With such an arrangement, however, the carriage is accelerated and decelerated repeatedly whenever it is scanned, as a result of which the load upon such components as the carriage driving motor increases. This raises the cost of the overall apparatus. Further, since the weight of the carriage per se is increased, problems such as vibration during printing arise.
One conceivable method of avoiding these problems is to refrain from providing the power-supply unit on the carriage and increase greatly the thickness of the wiring in order to lower wiring resistance. However, the larger the number of nozzles, the larger the surface area needed for routing of wiring on the circuit board, the larger the thickness required for the cables and the larger the size required for the connectors. This makes it difficult to lower the cost and reduce the size of the overall apparatus.
Accordingly, an object of the present invention is to provide a printing apparatus, as well as a method of controlling the power supply thereof, adapted so as to perform printing stably, even when the number of printing elements driven simultaneously changes, without enlarging the power supply and without reducing wiring resistance.
According to the present invention, the foregoing object is attained by providing a printing apparatus for performing printing on a print medium by a printhead having a plurality of printing elements, the apparatus comprising: a plurality of driving voltage sources of voltages that differ from one another; counting means for counting the number of printing elements that are to be driven simultaneously; and power supply selection means for selecting a driving voltage source to be connected to the printhead from among the plurality of driving voltage sources in accordance with the value of the count obtained from the counting means.
Further, according to the present invention, the foregoing object is attained by providing a method of controlling a power supply of a printing apparatus having a plurality of driving voltage sources of voltages that differ from one another, wherein the printing apparatus performs printing on a print medium by a printhead having a plurality of printing elements, the method comprising: a counting step of counting the number of printing elements that are to be driven simultaneously; and power supply selection step of selecting a driving voltage source to be connected to the printhead from among the plurality of driving voltage sources in accordance with the value of the count obtained at the counting step.
Thus, according to the present invention, the driving voltage source supplied to the printhead is selected in accordance with the number of printing elements to be driven at the same time, the selection being from among a plurality of driving voltage sources of different voltages.
This arrangement is such that if the number of printing elements driven simultaneously is large, for example, a driving voltage source for delivering a high voltage is connected to the printhead, thereby making it possible to drive each of the printing elements in a stable manner. A stable printing result is obtained, even when the number of printing elements driven simultaneously changes, without enlarging the power supply or reducing wiring resistance.
It is preferred that the plurality of driving voltage sources are branched from the same stabilized power supply circuit.
It should be noted that if the driving voltage sources, counting means and power supply selection means are provided on a carriage in a serial-scanning-type printing apparatus equipped with the carriage for holding a printhead and scanning the printhead in a direction that intersects the direction in which a print medium is transported, a fluctuation in the driving voltage is suppressed.
Further, it is preferred that the power supply selection means include classifying means for classifying count values into a plurality of categories.
The power supply selection means may be adapted in such a manner that one driving voltage source is connected to the printhead in accordance with the driving capability of each driving voltage source, or in such a manner that a plurality of driving voltage sources are connected to the printhead in accordance with the driving capability of each driving voltage source.
Furthermore, if it is so arranged that the power supply selection means does not connect a driving voltage source to the printhead when the value of the count from the counting means is zero, power consumption can be greatly reduced.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.